This invention relates to a ground-fault protective scheme for an electric distribution system and, more particularly, to a protective scheme of this type which is especially adapted for use with a three-phase four-wire system comprising two sources having neutral points that are interconnected by the fourth, or neutral, wire of the system.
Various ground-fault protective schemes have been developed for systems of this type, but typically they depend for proper operation upon there being a single connection between the neutral wire and ground. Examples of such protective schemes are shown in an article by Dunki-Jacobs et al in the magazine Industrial Power Systems, Volume 16, Issue 1, Mar. 1973, pages 15-19, published by General Electric Company at Schenectady, N.Y., and in U.S. Pat. No. 3,558,981--Zocholl. Also of interest is U.S. Pat. No. 3,259,802--Steen, assigned to the assignee of the present invention.
For improved safety, it is sometimes required that this type of system, instead of having a single common ground connection, have separate ground connections for the neutral point of each source located at or near such neutral point. The protective schemes described on pages 17-19 of the above-cited Industrial Power Systems article and in Zochnoll patent will not properly protect such a system since they depend upon all ground fault current returning to its source through a single ground connection.